1. Field of the Invention
The present invention relates to a method of applying a “Heading Then Merge Behind” guidance directive. It applies for example in the field of flight management systems.
2. Description of Related Art
A flight plan is the detailed description of the route that must be followed by an airplane within the framework of a planned flight. The flight plan comprises notably a chronological sequence of waypoints described by their position, their altitude and their overflight time. The waypoints constitute a reference trajectory to be followed by the pilot of the airplane if he wishes to best comply with his flight plan. But this reference trajectory is also a valuable aid to the ground control personnel. It allows notably the controller to anticipate the movements of the airplane and thus ensure the ASAS (“Airborne Separation Assurance Systems”) functions, for maintaining the inter-aircraft separation criteria. The flight plan is commonly managed aboard civil airplanes by a system referred to as the “Flight Management System”, which will be called the FMS subsequently. An FMS makes the reference trajectory available to the onboard personnel and to the other embedded systems.
Essentially with a view to safety, it is therefore necessary to ensure that the airplane follows at least in geographical terms and optionally in scheduling terms the reference trajectory described in the flight plan. For this purpose, guidance procedures make it possible to slave the airplane to the reference trajectory. For example, in so-called “managed” mode, the automatic pilot formulates maneuvers on the basis of the reference trajectory made available by the FMS and executes them automatically so as to follow as closely as possible in three-dimensional space the trajectory corresponding to the reference trajectory. However, in certain situations, it is preferable or indeed indispensable to veer from the reference trajectory. For example, the reference trajectory may cause the airplane to cut across another aircraft, violating the lateral separation criteria. From his ground control center, the traffic controller in charge of the flight perceives the risk in advance since he is cognizant of the whole of the aerial situation within a wide perimeter around the aircraft that he is controlling. He then implements pre-established ASAS procedures for coordination between the ground and the craft, these procedures commonly being grouped together under the name “Radar Vectoring”. “Radar Vectoring” procedures can make it possible to ensure that two aircraft cross under optimum safety conditions. They are based on a set of guidance instructions or “directives”, also predefined, that the controller passes to the pilot. Hereinafter in the present application, these guidance directives will be termed “ATC directives”, the acronym standing for “Air Traffic Control”. These directives are moreover commonly grouped together under the name “clearance ATC”. The pilot then disengages the functions of automatic following of the reference trajectory and manually executes the ATC directives that he receives from the controller, one after another, each time confirming their execution to the controller. Very often, the directives are exclusively passed verbally by VHF radio, the pilot confirming execution by phone also, although certain recent systems use data links in parallel to exchange ATC directives in the form of digitized messages.
For example, in order to maintain the criteria of lateral separation with another aircraft, the controller can pass the pilot an ASAS directive known as “Heading Then Merge Behind”, which will simply be called an “HTMB directive” hereinafter. In an approach zone or in a terminal zone, an HTMB directive serves notably to delay the alignment of an airplane with a landing runway, so as to control the temporal offset between two airplanes which share one and the same approach trajectory and which align themselves with one and the same runway. Since for these two airplanes, there is necessarily a convergence point with loss of spatial separation and risk of loss of temporal separation. An HTMB directive sent to the airplane that is supposed to land second makes it possible to guarantee a temporal separation between the two airplanes. This HTMB directive firstly indicates to the pilot which is “the target airplane” behind which he aligns himself, this target airplane necessarily being situated in proximity. It also indicates to the pilot a diversion heading to be followed temporarily, this diversion heading not corresponding to the envisaged heading on his reference trajectory. It also indicates to the pilot a rallying position for his reference trajectory, referred to as a “merge” point. Finally, it indicates to the pilot a separation in distance or in time to be complied with at the rallying position with respect to the target airplane behind which he aligns himself. With current FMSs, the pilot receiving an HTMB directive disengages the automatic pilot. Specifically, current FMSs do not ensure any ASAS functions such as the application of an HTMB directive. The pilot therefore manually follows the diversion heading indicated in the HTMB directive. It should be understood that this does not call into question the ASAS concept and the associated operational procedures, several levels of sharing of responsibilities between the ground controller and the pilot having been envisaged for implementing the ASAS procedures. But in certain cases, in addition to manually following the diversion heading, the pilot may also have to estimate, without assistance, the instant or position at which he ceases manually following this diversion heading so as to set a course heading towards the rallying position, it being absolutely essential that this instant or this position should make it possible to comply with the separation in distance or in time with respect to the target airplane. Manifestly, the work overload for the pilot and the stress generated are detrimental to flight safety. This is one of the technical problems that the present invention proposes to solve.